From what I can see in the ad, that looks like a nice bus. I would check thoroughly for rust. If it doesn't have rust issues I could get excited about it.
Reasonably low miles on a respected drivetrane. The 5.9 may be a tad under powered in a full size bus. There are folks here with 5.9's in full size buses that are quite happy with them.
I am partial to the bigger motors myself.
It appears to have an Allison 2000 or 3000 series transmission. That will get you 5 five forward gears with fifth being overdrive. And, the potential to have sixth gear unlocked giving you a second overdrive.
Regarding the price, I would venture a guess that the seller may well have bought the bus recently at auction for 2000-3000 and is flipping it. If you are comfortable with the idea of buying at auction you may find a similar deal.
If you prefer being able to test drive the bus and to be able to take it to a mechanic for inspection then buying from a dealer or private party makes sense. You will likely pay more if you go this route.
IMHO: Assuming no rust issues and a reasonable seller, I think $4500 would not be unreasonable from a private seller.